Unsaturated polyester resins are conventionally prepared by reacting dibasic acids; i.e., phthalic anhydride, maleic anhydride, etc., and difunctional alcohols; i.e., ethylene glycol, propylene glycol, etc., in stainless steel reactors at temperatures between 170.degree.-200.degree. C. Corrosion of stainless steel is slow enough in these systems that it does not have an adverse effect on either reactor life or resin color.
Synthesis of unsaturated polyester resins such as those described in U.S. Pat. No. 3,507,933, i.e., fire retardant resins prepared from phthalic anhydride, maleic anhydride, and dibromoneopentyl glycol, results in corrosion problems when carried out in stainless steel reactors, and this in turn leads to color problems.
The use of sulfuric acid and sulfonic acids as catalysts for polyesterification reactions using saturated dibasic acids are known in the prior art, e.g., P. J. Flory disclosed the use of p-toluene sulfonic acid (p-TSA) as a catalyst for the reaction of adipic acid and ethylene glycol in 1939. Later attempts to use strong acid catalysts in the preparation of unsaturated polyester resins have been less than successful. Weakly acidic or basic catalysts are usually required for reactions involving aliphatic diols in order to prevent dehydration to ether or olefin. It has also been pointed out that acid catalysts such as sulfuric acid or p-TSA, while increasing the rate of both esterification and isomerization, usually cause color formation and other detrimental side reactions.
Some of these adverse reactions caused by the presence of sulfuric, or sulfonic acids, include premature gelation of the styrenated and unstyrenated resins and the loss of glycol from the cook due to the formation of volatile ethers. It has been shown that as little as 50 ppm of sulfuric acid will catalyze the formation of ethers from aliphatic diols at polyester reaction temperatures.